The present invention relates to a system for measuring an intake airflow rate in an engine for a motor vehicle, in order to determine the fuel injection time and the ignition timing.
In a known fuel injection system, an airflow meter is provided in an intake passage at a position upstream of a throttle valve to detect the intake airflow rate (Q). The fuel injection time Tp is determined by a calculation of Tp=Q/N (N is engine speed).
Further, the ignition timing is also determined by using the intake airflow rate. Accordingly, a high accuracy is required in measuring the intake airflow rate. However, in the prior art, it is difficult to accurately measure the intake airflow rate as described hereinafter.
Since the airflow meter is positioned upstream of the throttle valve, the intake air induced into a cylinder of the engine is not directly measurable. Namely, there is a space between the throttle valve and the cylinder, such as a chamber formed downstream of the throttle valve and an intake manifold. Accordingly, when the throttle valve is rapidly opened, the induced air enters into the space to increase the pressure in the space. In other words, an amount of air sufficient to increase the pressure in the space is included in the induced air through the throttle valve. The airflow meter measures the intake airflow rate including the air which is not induced into the cylinder. Accordingly, the measured intake airflow rate increases temporarily. If the injection time is calculated based on the increased intake airflow rate, the fuel injector supplies a larger quantity of fuel than the necessary quantity, thereby enriching the air-fuel mixture. The enrichment of the mixture causes a reduction of the output of engine and increases of noxious emissions such as CO and HC. Further, if the ignition timing is calculated based on the increased intake airflow rate, the timing deviates from a desired timing.
Similarly, when the throttle valve is closed, the air-fuel ratio and ignition timing deviate from desired values.